This invention relates to a timing-phase control system provided on the receiving side of a duobinary transmission system, and in particular to a timing-phase control system for duobinary transmission adaptable to high-speed PCM repeater systems using a wide-band transmission medium such as a coaxial cable.
Repeaters provided on PCM transmission lines generally have three basic functions known as the three Rs, i.e. reshaping, retiming and regeneration of received waveforms. Stated in more detail, such repeaters are required to discriminate equalized waveforms in an appropriate timing phase, thereby regenerating the original signals sent out by a transmitter. An incorrect setting of the timing phase at the repeaters will result in erroneous discrimination and deterioration of the quality of transmission.
In conventional PCM transmission systems, a timing wave needed for retiming is extracted by processing the received wave by the use of a non-linear circuit. This method is known as the self-timing system resorting to non-linear extraction, and has the advantage that the phase relationship of the extracted timing wave and the equalized waveform is relatively stable due to invulnerability to the fluctuation of the transmission characteristics which is achieved by using an appropriate equalization method. However, this non-linear extraction is not necessarily an effective method for the duobinary transmission to be described below.
One of the attempts to use a duobinary transmission system for PCM transmission is proposed by Y. Higo and A. Sawai in "Experimental Study on 800 Mb/sec Duobinary Coaxial PCM Transmission with DC Restoration Technique" published in the Conference Record of the 1976 International Conference on Communications held in Philadelphia, Pa., June 14-16, 1976, pp. 6-11 through 6-16 (Reference 1). In the duobinary transmission, a single pulse supplied from the transmitter is equalized into a wide pulse spanning two time slots, and consequently takes a waveform in the form of a 0110... pattern as viewed at the timing points. Accordingly, if binary information represented by "0" or "1" is sent out from the transmitter, the equalized output on the receiving side will become ternary information represented by "0", "1" or "2" because of the overlapping between pulses. By subjecting the received ternary information to suitable code conversion processing, the original binary information is obtained. For details of this code conversion technique, reference is made to the article entitled "Correlative level coding for binary-data transmission", by Adam Lender published in IEEE Spectrum, February, 1966, pp. 104 to 115 (Reference 2).
Such duobinary transmission is an effective method for efficient utilization of the frequency band because it can transmit information having the same pulse repetition rate within about one half of the frequency band needed in the conventional PCM transmission. However, owing to the narrower transmission band in the duobinary transmission, the extraction of timing information becomes difficult by a simple application of the known non-linear extraction technique.
To solve this problem, in the system described in Reference 1, a pilot signal serving as the timing information is transmitted from the transmitter superimposed on the transmission signal and is extracted by a tank circuit or the like provided at the receiver. This technique is known as the external timing method. However, unlike the above-mentioned self-timing method using non-linear extraction, the external timing method has the disadvantage that a relative deviation in the discrimination timing phase tends to occur due to variation in pilot phase setting at the transmitter as well as phase changes in transmission characteristics.